I started in 1985, with my earliest memories being the issue of New Titans where Dick retired the Robin identity (and, over in Batman, passed it on to a new kid named Jason Todd) and Wally left the team, and the Green Lantern story arc where Hal had been replaced as Earth's Lantern by John Stewart and was fighting without a ring to reestablish his place in the Corps. The latter story was what the Green Lantern title was up to during the Crisis on Infinite Earths, and continued with the Green Lantern title being rebranded as Green Lantern Corps and following the exploits of a team of seven Earth-based Lanterns (even though only two of them were human). And while I had seen one or two Superman comics before then, it was just enough for me to recognize that John Byrne's relaunch of Superman was changing things for him. Meanwhile, Barry Allen died in the Crisis and Wally took over the Flash identity.
So I got into the post-Crisis DCU on the ground floor… and then life happened and I dropped out of comics again until 1992, with the Death of Superman and Knightfall, and Tim Drake flying solo with the first ever Robin ongoing series. That, in turn, lead to me backtracking a bit and finding out about how Tim had become Robin and what had happened to that Jason kid I had briefly read about seven years earlier. Emerald Twilight lead to Kyle Rayner becoming Green Lantern. After Zero Hour, I got into the rebooted Legion of Superheroes as well, but also Starman. And of course, it wasn't long until Ollie died and we were introduced to Connor Hawke.
My early experience with comics was all about change, and I doubt that I would have been drawn in by DC if they had remained as they had been in the early 80s. Even when Morrison brought back the “big seven” JLA near the end of the decade, out was a mixture of old and new, with Wally and Kyle fighting alongside Clark, Bruce, and Diana; and that notion continued with Young Justice and the return of both the Titans and the JSA: Starman had had its impact, and I was now rooting for the legacy concept.
Then Dan Didio came along. YJ and Titans were savaged by the poorly-named Graduation Day; between Wargames and Identity Crisis, Tim Drake took it on the chin; and then there was Infinite Crisis, which had as its premise that everything I had been following for the last decade was an unrelenting spiral of darkness that had to be stopped by the return of the original Superman… only for him to be killed and for an actual darkening of the stories that had started in 2003 to accelerate through the next decade-plus. Life happened again, this time nicely synchronized with Flashpoint which made New 52 a great jumping-off point for me; and I didn't really come back until Convergence and Multiversity. I followed Superman: Lois & Clark and Titans Hunt during the “DCYou” year, and was all in when DCU Rebirth provided to bring back much of what New 52 had jettisoned; though it felt more like “damage control” than an actual rebirth.
At this point, I've become accustomed to a roller-coaster ride, where everything seems to be a tug-of-war between developers with very different, and largely incompatible, ideas as to where to go next. DC is now damaged goods, and I'm not sure it can recover.
So to answer the original question: to me, the major event in DC comics history that caused the most damage was Titans/YJ: Graduation Day. That was the point that Dan DiDio truly took over control at DC and started tearing down everything I loved about the DCU.